This invention relates generally to nuclear reactors and more particularly, to assemblies and methods for coupling core spray line assemblies within such reactors.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure.
Boiling water reactors have numerous piping systems, and such piping systems are utilized, for example, to transport water throughout the RPV. For example, core spray piping is used to deliver water from outside the RPV to core spargers inside the RPV and to cool the core. Typically, the core spray piping is coupled to a thermal sleeve which is welded to a RPV nozzle, or safe end.
Stress corrosion cracking (SCC) is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high temperature water. The reactor components are subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. In addition, water chemistry, welding, heat treatment and radiation can increase the susceptibility of metal in a component to SCC.
Reactor internal piping, such as thermal sleeves and core spray lines, occasionally requires replacement as a result of SCC. Replacing the core spray piping often requires removing the core spray line from the RPV nozzle safe end. Because the spray line is welded to the safe end, removing the spray line from the safe end often damages, or breaks, the safe end. To replace the safe end, the reactor must be drained to an elevation below that of the safe end. The safe end is then cut off and a replacement is welded to the RPV. Thereafter, the replacement core spray line is welded to the replacement safe end. Replacing a safe end is time consuming and tedious.
It would be desirable to provide an assembly which facilitates replacing core spray lines without removing the reactor pressure vessel safe end. It also would be desirable to provide such an assembly which is easily removed and installed without the necessity of welding.
A T-box assembly, for attachment to a core spray nozzle safe end in a boiling water nuclear reactor pressure vessel, in an exemplary embodiment, includes a T-box housing, a thermal sleeve coupled to the T-box housing, a cruciform wedge configured to engage an inside surface of the bore of the core spray nozzle safe end, a spider threadedly engaging the thermal sleeve, and a draw bolt coupling the cruciform wedge and the spider. At least one spring washer is located between the end of the thermal sleeve and the end of the core spray nozzle safe end. The T-box housing includes three ends. The first end is coupled to the thermal sleeve, and the other two ends are configured to be in substantial alignment with each other and couple to core spray line header pipes. The T-box housing also includes a cover opening sized to receive a cover plate, and that is in substantial alignment with the first end. T-box housing further includes a plurality of breach lock lugs extending into the cover opening.
The T-box cover plate includes a head portion and an insertion portion. The insertion portion includes a plurality of breach lock lugs extending from the periphery of the insertion portion. The insertion portion breach lock lugs are sized and located to mesh with the T-box housing breach lock lugs to capture the cover in place. A plurality of ratchet teeth extend around the periphery of the head portion.
The T-box housing further includes a recess located adjacent the cover opening. The recess is sized and shaped to receive a lock spring. The lock spring includes an engagement portion having a plurality of ratchet teeth sized to mesh with cover plate ratchet teeth.
The cruciform wedge includes a central member having a bore extending therethrough. Four web members extend from the central member. The web members are configured so as to form an X shaped configuration. Two support members extend between the ends of adjacent web members with a first support member extending between the first and second web members, and a second support member extending between the third and fourth web members. The support members engage the bore of the core spray nozzle safe end.
The draw bolt includes, at one end, a cap portion having a larger diameter than the diameter of the bore through the cruciform wedge central member. The other end is threaded and sized to threadedly engage a draw bolt nut.
The spider includes a cylindrical shell with one end portion configured to engage the inside surface of the nozzle safe end, and the other end portion having threads sized to threadedly engage the thermal sleeve. A plurality of vanes extend from an inside surface of the spider shell to a spider center member. The spider center member includes an axial bore therethrough. The spider bore and the cruciform wedge bore are aligned and sized to receive the draw bolt.
The above described T-box assembly facilitates replacing the core spray line without removing the core spray nozzle safe end or draining the reactor. In addition, the T-box assembly facilitates attaching the core spray line to the safe end without welding.